With one strengthening and rebuilding project in Tianjin city as background,external prestressing test was conducted on 12 beams and frame beams with 6 Pin,the stiffness and crack resistance of beam body after strengthening were analyzed.At the same time normal section flexural bearing capacity calculation formula and ultimate stress of tendon incremental calculation formula suitable in this project were confirmed.The results showed that reinforced concrete beams with external prestressing could reduce significantly beam deflection and crack width,and improve significantly ultimate bearing capacity of beam bending with limit state capacity.And fitness beam ductile failure was still the failure mode of beam after strengthening design with proper prestressing tendon.